1. Field of the Invention
The present invention relates to a vehicle dynamics control system, and specifically to a system which is capable of automatically controlling or compensating for the vehicle's cornering behavior or vehicle's turning behavior or steer characteristics such as understeer or oversteer on turns by automatically properly controlling or regulating the hydraulic brake pressure applied to the individual wheel-brake cylinders.
2. Description of the Prior Art
As is generally known, an automotive vehicle has front-left, front-right, rear-left and rear-right wheel-brake cylinders respectively connected to a master cylinder where hydraulic pressure is developed by depression of a brake pedal. In recent years, the hydraulic brake system has been split into two independent sections, so that, if one brake section fails owing to damage or brake-fluid leakage, the other section will provide braking. Such a hydraulic brake system is called a "dual brake system". A dual-brake system master cylinder has two pistons, set in tandem. The brake pedal (the foot pedal) actuates the two master-cylinder pistons and forces brake fluid along respective piping systems to individual wheel-brake cylinders (operating cylinders). Front-engine, rear-wheel-drive (FR) vehicles, generally use a so-called parallel split layout of brake circuits, in which one part of the tandem master cylinder output is connected via a first brake pipeline system to front-left and front-right wheel-brake cylinders and the other part is connected via a second brake pipeline system to rear-left and rear-right wheel-brake cylinders. In automobiles with such a parallel split layout of brake circuits, when the brake pedal is depressed by the driver and thus primary and secondary master-cylinder pistons are pushed, the brake-fluid pressure generated from one part of the master cylinder output and the brake-fluid pressure generated from the other part are supplied respectively via the first and second brake pipeline systems to front and rear wheel-brake cylinders, with the result that the negative wheel torque (resulting in a braking force) is applied to the individual wheels. As is generally known, when a vehicle is rounding a curve, owing to road surface conditions (so-called low-.mu. or high-.mu. roads), changes in the vehicle velocity, throttle-on or throttle-off conditions, or the like, the vehicle may often exhibit undesired steer characteristics, namely oversteer tendencies in which the actual radius of turn is less than the intended radius of turn or understeer tendencies in which the actual radius of turn is greater than the intended radius of turn. Oversteer is generally known as an under-response to steering input as by generation of excessive slip angle on rear road wheels, whereas understeer is generally known as an over-response to steering input as by generation of excessive slip angle on front road wheels. The driver must have a great deal of skill to avoid undesired understeer or oversteer by adjusting increase or decrease in steer angle only by way of the driver's braking or steering action. For the reasons set forth above, in recent years, there have been developed and proposed various active steer-characteristics control systems in which the vehicle's cornering behavior is automatically controlled or regulated by adjusting the brake-fluid pressure applied to each individual wheel-brake cylinder by means of an electronic control unit (ECU) or an electronic control module (ECM). One such vehicle's cornering behavior controller (simply a vehicle controller) has been disclosed in Japanese Patent Provisional Publication No. 8-133039. In the vehicle controller disclosed in the Japanese Patent Provisional Publication No. 8-133039, when the vehicle experiences understeer during turns, the vehicle controller operates to reduce the brake-fluid pressure in the wheel-brake cylinder of the front road wheel rotating on the outside and simultaneously to build up the brake-fluid pressure in the wheel-brake cylinder of the rear road wheel rotating on the inside, by way of an automatic control, thereby avoiding understeer. On the contrary, when the vehicle experiences oversteer on turns, the vehicle controller operates to build up the brake-fluid pressure in the wheel-brake cylinder of the front road wheel rotating on the outside and at the same time to reduce the brake-fluid pressure in the wheel-brake cylinder of the rear road wheel rotating on the inside, by way of an automatic control, thereby avoiding oversteer. As previously discussed, the Japanese Patent Provisional Publication No. 8-133039 teaches the increase in brake-fluid pressure in the rear wheel-brake cylinder on the inside to avoid understeer during turns. However, on turns, the car weight is usually transferred to the outside front wheel. Thus, the magnitude of wheel load acting on the inner rear wheel tends to become the minimum value during turns, as compared with the other road wheels. For the reasons discussed above, even if, on turns, the brake-fluid pressure in the rear wheel-brake cylinder on the inside is built up by means of the conventional vehicle controller, it may be impossible to carry out adequate braking effect, and thus it may be difficult to effectively avoid understeer tendencies owing to load transfer to the outer front wheel during turns. Additionally, in front-engine, rear-wheel-drive (FR) vehicles, a so-called parallel split layout of brake circuits is generally used. Assuming that the system disclosed in the previously-noted Japanese Patent Provisional Publication No. 8-133039 is applied to a front-engine, rear-wheel-drive vehicle with a parallel-split layout of brake circuits, the front-section brake pipeline system and the rear-section brake pipeline system must be both disconnected from the dual master cylinder once and in lieu thereof the two brake pipeline systems must be connected to the respective hydraulic pumps serving as an external fluid-pressure source, while the automatic control is executed with regard to both the outer front road wheel and the diagonally-opposed inner rear road wheel under the understeer or oversteer state condition on turns. Therefore, while the automatic control is actually executed to avoid understeer or oversteer on turns, the master-cylinder pressure cannot be directed to the respective wheel-brake cylinder, and thus the vehicle cannot be decelerated according to the driver's wishes (or the magnitude of the driver's foot pedal depression).